There is already known an ion generator which applies a high voltage from an AC high voltage source of the commercial frequency (50 or 60 Hz) between a discharge needle and an opposed electrode, generates a corona discharge from the discharge needle and ionizes air by that corona discharge (see Japanese Patent Application Laid-Open No. 8-288094 for instance).
In an ion generator of this kind, positively charged air ions and negatively charged air ions are alternately generated by alternately applying an AC voltage to the discharge needle. And the ion generator of this kind, as it can neutralize the electric charges (static electricity) accumulated on the charged object with the generated positive and negative air ions, is generally used as a deelectrifying device for clearing charged objects of static electricity.
Further, a consideration is given in the ion generator of this kind to the short-circuiting current which may be generated when a human body or the like comes into contact with the discharge needle, and the short-circuiting current is restrained by capacitance-coupling the discharge needle with the high voltage output line from the AC high voltage source. In the ion generator in this case, at the time of generation of a corona discharge (at the time of discharge by the discharge needle) the impedance of the coupled capacitance of the discharge needle causes the discharge needle to reduce the voltage of the high voltage output line. In order to generate a corona discharge at the commercial frequency, the discharge needle requires a voltage of about 4 kV at its tip. For this reason, this ion generator uses an AC high voltage power source which outputs a high voltage, augmented with a compensation for the voltage drop due to the impedance of the coupled capacitance of the discharge needle, to the high voltage output line.
It is difficult here for the discharge needle to have a very large coupled capacitance because of structural constraints and the need to secure the effect to restrain the short-circuiting current, and the capacitance can be at most 10 pF or so for practical purposes. As a result, the voltage drop attributable to this coupled capacitance increases. In a case in which coupled capacitance is 10 pF and the commercial frequency is 50 Hz, the voltage drop will reach about 1.6 kV. Incidentally, the discharge current of the discharge needle is about 3 μA to 10 μA, and the above-mentioned level of the voltage drop is what matches a discharge current of 5 μA. Therefore, in order to compensate for this voltage drop, the conventional ion generator uses as the boosting transformer for the AC high voltage power source a wound-wire transformer having a sufficient number of windings to generate a high voltage of about 6 to 9 kV. However, since a wound-wire transformer is relatively large and heavy, this involves a problem of difficulty to make the ion generator compact and light.
On the other hand, there is also known an ion generator using a piezoelectric transformer, which is more compact and lighter than a wound-wire transformer and an AC high voltage power source of a high frequency of a few tens of kHz instead of the commercial frequency (see Japanese Patent Application Laid-Open No. 2003-22897 for instance). The AC high voltage power source of this ion generator generates a high frequency AC high voltage by providing a high frequency signal of a few tens of kHz from a high frequency oscillator to the piezoelectric element of the piezoelectric transformer. An ion generator using such a high frequency power source, compared with what uses a power source of the commercial frequency, can improve the ion balance of air ions (the balance between the quantity of positive ions and that of negative ions), and moreover can reduce the voltage needed for generating a corona discharge from the tip of the discharge needle to about 1.8 kV.
The output voltage of the high frequency power source using this piezoelectric transformer is at most about 2 to 3 kV because of the characteristics of the piezoelectric transformer, and this output voltage is close to the voltage (about 1.8 V) needed by the discharge needle to generate a corona discharge by using that high frequency power source. Therefore, in order to secure the voltage of the discharge needle at a level allowing the generation of a corona discharge, the voltage drop from the high frequency power source to the discharge needle has to be kept sufficiently small. As the current a piezoelectric transformer can output is generally small (at most about 100 μA), the short-circuiting current can be kept sufficiently small without having to capacitance-couple the discharge needle with the high voltage output line.
On account of these circumstances, in the conventional ion generator using a high frequency power source, the high voltage output line is directly connected to the discharge needle (the discharge needle is not capacitance-coupled with the high voltage output line) so that no superfluous voltage drop may occur between the high voltage output line of the high frequency power source and the discharge needle.
Incidentally, the requirement for neutralizing charged objects wherever practicable on the production lines of precision semiconductor devices and elsewhere has become even more stringent in recent years. In meeting this requirement, an ion generator using a high frequency power source is more advantageous than an ion generator using a commercial frequency power source. However, in the conventional ion generator using a high frequency power source, the ion balance is often destabilized, and the requirement cannot be always fully satisfied.
An object of the present invention, attempted in view of these background circumstances, is to provide an ion generator reduced in the size and weight of hardware configuration and capable of improving the balance between positive and negative air ions and its stability.